If the Greens thought the past three years were challenging, just wait for the next three. They – and Labour – need to figure out a new way of growing the centre-left bloc without tearing each other to pieces

If her speech is anything to go by, it was a confident and combative Metiria Turei who took to the stage at the Greens' policy conference in Palmerston North yesterday, looking over what the new Greens had created and declaring "it was very good". And why not? The Greens were arguably the smartest political operators last term, increasing their support by two-thirds and bringing seven new MPs into the House.

Green co-leader Turei rightly boasted:

"We have survived leadership changes and the retirement of all of our original Green MPs, and we are still here and stronger than ever."

Many in political circles were dubious of the second generation of Greens and their ability to build on the foundations laid by the much-admired Rod Donald and Jeanette Fitzsimons. But that's just what they did.

So credit where credit's due. But let's also not get too giddy about the Greens' future prospects; however well they've done reaching base camp and pitching their tents, she's a hard road yet to the top.

And it seems the top's where they want to be. Turei spoke of not being Labour's "little brother" and "being bigger, stronger and fiercer than ever". She said

"This election does not represent a high water mark for Green support. It is just the beginning. We will get bigger and stronger."

Which can only be read as a direct challenge to Labour's ascendancy on the centre-left. Or so you'd think.

Turei went on to say:

"We are not the benefactors of Labour or National voters shifting to the Greens. We are our own political force that generates support in our own right."

To which I'd ask, so where do you think your increased support came from? The pixies? Santa Claus? The horribly low turn-out at the election makes if crystal clear the Greens aren't adding to the sum total of voters, so of course it's stealing votes from the big parties, Labour in particular.

Turei's claim to the contrary is rhetorical nonsense. But it does raise a crucial point of tension that will be well worth watching between now and 2014.

Fact is, the number of voters is finite. While there's always room to bring in non-voters, by and large politics is a competition for market share. In their more sober moments, Turei and her team must recognise that their peak coincided with Labour's nadir, and that's no coincidence. Labour's survival depends on it also getting "bigger and stronger".

So the real question for both parties is how to grow the combined centre-left vote, rather than cannibalising each other or getting into a self-destructive slagging match for a percentage of the vote that won't even get you into the Beehive.

It hardly needs saying that most of that growth has to come from the 'centre' part of that combo, rather than the 'left'. And that means taking votes off National.

Turei implied that she understood as much by ripping into John Key as a wheeler-dealer Prime Minister whose government is dishing out tax dollars to big business whilst cutting the state sector and ignoring the poor. She couldn't have been clearer, saying:

"It is our goal is to remove this Government; to replace it with one of our own construction."

(It's a fascinating sentence, that one. It's quite a hostile line for the peace-loving Greens. It doesn't talk of winning over the public, but of simply eliminating the opposition. It lays out the Greens' ambition to be at least Kingmaker, and ultimately even the senior party of the left.)

The Greens recognised the need to win some in the centre by last year opening the door to a deal with National, even if they repeatedly described it as "highly unlikely". It fenced National off (a little) and helped shake off some of the 'loony left' perceptions. But I'm not sure if they can go back to that well. To use a terribly un-Green metaphor, they will have to drill somewhere else this term.

It helps that the party's on the right side of "centrist" public opinion on the likes of asset sales, overseas ownership and probably even deep sea drilling helps, but a more sophisticated strategy will be needed.

For a start, the attacks from the right are likely to be bigger and stronger too. It's not hard to imagine what National – and even the Maori Party – make of other lines in the speech when Turei promises to "to engage constructively" with the government and find common ground: "You want to work with us while declaring your desire to destroy us? Go take a running jump".

But the more interesting and vital question is how Labour responds to the Greens this term. Labour has undoubtedly lost support to the Green Party. So does it go for them hammer and tongs? Publicly new leader David Sheaer is up for a fight, saying:

"We're not going to sit back and see the Green Party eat away our support."

Which must delight National no end. It would love to see the centre-left parties attacking themselves, much as the Democrats are loving the spectacle of the Republican candidates tearing strips off each other in the US primaries.

When Gingrich calls Romney a liar, he's simply doing Barack Obama's work for him – and in a much more damaging way, because it's criticism by a supposed friend rather than enemy. The only winner is the other guy.

No, Labour and the Greens need to nut out a new way. Does David Shearer – he of the life and death negotiations in refugee camps – have a different strategy behind the scenes? Does he have it in him to be a true MMP leader? (if anyone even knows what that means).

The political fact is that Labour and the Greens combined can probably count on around 40 percent of the vote; they need to find at least five percent more in the next five years. Where does that come from? And which party gets it?

Does Labour need to work harder with the Greens to find (gift) them a winnable electorate seat? Does it need to back-off some popular issues to allow the Greens a positive spotlight? Do the Greens need to talk up Shearer's credentials as a PM? Can personal and policy bridges be built by the likes of Green-friendly Labour front-bencher David Parker?

I'm not sure of the detail, but if they can't find a way of working together while holding to what makes them distinct and different parties, National will be able to feel more confident about a third term.